Xerographic fixing apparatus



Jan. 29, 1963 J. J. CODICHINI ETAL XEROGRAPHIC FIXING APPARATUS 3 Sheets-Sheet 1 Filed Dec. 23, 1960 INVENTORS. J-OSEPH J.COD|CHIN| BY WILLIAM G. LEWIS ATTORNEY 1963 J. J. CODICHINI ETAL 3,076,033

XEROGRAPHIC FIXING APPARATUS Filed Dec. 23, 1960 3 Sheets-Sheet 2 INVENTIORS. JOSEPH J. comm-um BY WILLIAM GLEWIS ATTORN EY 1963 J. J. CODICHINI ETAL 3,076,033

XEROGRAPHIC FIXING APPARATUS Filed Dec. 25, 1960 5 Sheets-Sheet 3 SOL-I INVENTORS. JOSEPH J.COD|C HIN| WILLIAM G. LEWIS ATTO R NEY United States Patent Ofiiee 3,076,083 Patented Jan. 29, 1963 3,076,083 XEROGRAPHIC FIXING APPARATUS Joseph J. Codichini, Fairport, and William G. Lewis,

Rochester, N.Y., assignors to Xerox Corporation, a corporation of New York Filed Dec. 23, 1960, Ser. No. 77,952 4 Claims. (Cl. 219-19) This invention relates to improvements in heat fusers and, particularly, to an improved apparatus for heat fixing of xerographic powder images.

More specifically, the invention relates to an improved heat fuser for use in the field of xerography to fuse resinous powder images on a support material.

In the process of xerography, for example, as disclosed in Carlson Patent 2,297,691, issued October 6, 1942, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them, and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely divided developing material or toner which is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferred to a support surface such as paper to which it may be fixed by any suitable means.

One of the methods in common use for developing the electrostatic latent image is described in Walkup Patent 2,618,551 and is known as cascade development, and is i general use for line copy development. In this tech nique, the powder or toner is mixed with a granular carrier material, and this two-component developer is poured or cascaded over the plate surface. The function of the carrier material is to improve the flow character-- istics of the powder and to produce, on the powder, by triboelectrification, the proper electrical charge so that the powder will be attracted to the image. More exactly, the function of the carrier material is to provide the mechanical control to the powder, or to carry the powder to an image surface and, simultaneously, to provide homogeneity of charge polarity.

In the Carlson patent it is noted that a variety of types of finely divided electroscopic powders may be employed for developing electrostatic latent images. However, as the art of xerography has progressed, it has been found preferable to develop line copy images with a powder or toner formed of any of a variety of pigmented thermoplastic resins t-hat have been specifically developed for the purpose. A number of such developing materials are manufactured and marketed by Haloid Xerox Inc. of Rochester, New York, and are specifically compounded for producing dense images of high resolution and to have characteristics to permit convenient storage and handling. Such developing materials are compound to permit them to be fixed to the surface of a transfer material either by heat fixing or vapor fixing techniques, in accordance with the particular application in which they are employed; that is, the individual particles of resin (toner) soften and coalesce when heated or plasticized by solvent, so that they become sticky and readily adhere to the surface of the transfer material.

One of the important applications of the process of xerography comprises its use in automatic copying machines for general office use wherein the powder images formed on a xerographic plate are transferred to cut sheet paper and then fixed thereon by heat fusing. In order to fuse resinous powder images formed of the powdered resins now commonly used, it is necessary to heat the powder and the paper to which it is to be fused to a relatively high temperature, such as approximately 325 F. It is undesirable, however, to raise the temperature of the paper substantially higher than 375 F. because of the tendency of paper to discolor at such elevated temperatures.

This imposes rather exacting temperature control requirements on the fusing device and also makes it necessary that the heat be distributed uniformly over the surface area of the support material to be treated. A further requirement is that no portion of the support material should accidentally come into contact with the heating element, or if it comes into contact with the heating element the heating element shall have a temperature that will not damage the support material.

Other requirements are that the heat fuser shall provide good and rapid heat transfer to the sheet 'of support material being treated, that it shall have relatively low power consumption when left on for extended periods'of time, such as when it is maintained in standby condition, and that heat dissipation from the heat fuser shall be kept at a minimum to prevent possible damage to the xerographic plate of the xerographic reproducing apparatus.

It is therefore an object of this invention to improve heat fusing apparatus to satisfy the above-stated fusing requirements.

Another object of this invention is to improve heat fusers to provide uniform heating of a support material carrying a fusible powder image thereon.

It is a further object of the invention to improve xerographic fixing apparatus for fusing large surface areas of a support material and to accomplish the fixing free from danger of'charring or other damage to the support material.

For a better understanding of the invention as well as other objects and further features thereof, reference is had-to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein:

FIG. 1 illustrates schematically an automatic xerographic reproducing apparatus using a preferred embodiment of a heat fuser of the invention;

FIG. 2 is a side view of a heat fuser of the invention taken on the inlet side of the fuser;

FIG. 3 is a top view of the heat fuser apparatus;

FIG. 4 is an end view of a heat fuser; and

FIG. 5 is a sectional view of the heat fuser taken along line 5-5 of FIG. 4.

General As shown, the xerographic apparatus comprises a xerographic plate including a photoconductive layer or lightreceiving surface on a conductive backing and formed in the shape of a drum, generally designated by numeral 20, which is journaled in a frame to rotate in the direction indicated by the arrow to cause the drum surface sequen tially to pass a plurality of xerographic processing stations For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the drum surface may be described functionally as follows:

A charging station, at which a uniform electrostatic charge is deposited on the photoconductive layer of the xerographic drum; 7

An exposure station, at which a light or radiation pattern of copy to be reproduced is projected onto the drum surface to dissipate the drum charge in the exposed areas thereof and thereby form a latent electrostatic image of the copy to be reproduced;

A developing station, at which a xerographic developing material including toner particles having an electrostatic charge opposite to that of the electrostatic latent image are cascaded over the drum surface, whereby the toner particles adhere to the electrostatic latent imagine to form a xerographic powder image in the configuration of the copy to be reproduced;

A transfer station, at which the xerographic powder image is electrostatically transferred from the drum surface to a transfer material or support surface; and

A drum cleaning and discharge station, at which the drum surface is first charged and then brushed to remove residual toner particles remaining thereon after image transfer, and at which the drum surface is exposed to a relatively bright light source to effect substantially complete discharge of any residual electrostatic charge remaining thereon.

The charging station is preferably located as indicated by reference character A in the schematic illustration of the apparatus. In general, the charging apparatus or corona charging device 21 includes a corona discharge array of one or more discharge electrodes that extend transversely across the drum surface and are energized from a high potential source and are substantially enclosed within a shielding member.

Next subsequent thereto in the path of motion of the xerographic drum is an exposure station B. This exposure station may be one of a number of types of mechanisms or members such as desirably an optical scanning or projection system or the like designed to project a line copy image onto the surface of the photoconductive xerographic drum from a suitable original.

The optical scanning or projection assembly consists of a copyboard in the shape of a drum, hereinafter referred to as copy drum 22, which is adapted to support copy to be reproduced and arranged to rotate in light-projection relation to the moving light-receiving surface of the xerographic plate. Uniform lighting is provided by suitable lamps attached to a slotted light reflector 23 mounted adjacent to the copy drum.

A light shield 24 adapted to protect the xerographic plate from extraneous light is positioned adjacent to the surface of the xerographic plate. A slot aperture 25 in the light shield extends transversely to the path of movement of the light-receiving surface of the xerographic drum 20 to permit reflected rays from the copy drum to be directed against a limited transverse area of the light-receiving surface as it passes therebeneath.

To enable the optical system to be enclosed within a relatively small cabinet, a folded optical system including an object mirror 26, a lens 27, and an image mirror 28 is used in the preferred embodiment of the apparatus.

Copy fed through paper guides 31 to the copy drum is removably secured thereon by a suitable gripper mechanism for movement therewith in timed relation to the movement of the xerographic drum whereby a flowing image of the copy is projected onto the xerographic drum. The copy is held against the surface of the copy drum by means of guides 32 and 33, the latter also preventing the trailing edge of the copy from contacting the web cleaner 54. After'the copy is scanned it can be released from the copy drum to be transported out of the machine by copy feed out roller 34 coacting with the peripheral surface of the copy drum to forward the copy through copy guide 30.

Adjacent to the exposure station is a developing station C in which there is positioned a developer apparatus 35 including a developer housing having a lower or sump portion for accumulating developer material 36. Mounted within the developer housing is a motor driven bucket-type conveyor used to carry the developer material previously supplied to the developer housing to the upper portion of the developer housing from where the developer material is cascaded over a hopper chute onto the drum.

As the developer material cascades over the drum, toner particles of the developer material adhere electrostatically to the previously formed electrostatic latent image areas on the drum to form a visible xerographic powder image; the remaining developer material falling off the peripheral surface of the drum into the bottom of the developer housing. Toner particles consumed during the developing operation to form the xerographic powder images are replenished by a toner dispenser 37, of the type disclosed in copending application Serial No. 77,955, filed concurrently herewith on December 23, 1960, in the name of Roger H. Eichorn and William G. Lewis, mounted within the developer housing.

Positioned next adjacent to the developing station is the image transfer station D which includes suitable sheet feeding mechanism adapted to feed sheets of paper successively to the xerographic drum in coordination with the presentation of the developed image on the drum at the transfer station. The sheet feeding mechanism includes a sheet source such as tray 41 for a plurality of sheets of a suitable support material, that is, sheets of paper or the like, a separator roller 42 adapted to feed the top sheet of the stack of support material to a sheet conveyor mechanism 43 having paper grippers 44 thereon which carry the sheet support material into contact with the rotating xerographic drum in coordination with the appearance of a developed image at the transfer station.

The transfer of the xerographic powder image from the drum surface to the support material is eifected by means of a corona transfer device 45 that is located at or immediately after the point of contact between the support material and the rotating xerographic drum. The corona transfer device 45 is substantially similar to the corona,

discharge device that is employed at the charging station in that it also includes an array of one or more corona discharge electrodes that are energized from a suitable high potential source and extend transversely across the drum surface and are substantially enclosed with a shielding member. In operation, the electrostatic field created by the corona transfer device is effective to tack the transfer material electrostatically to the drum surface and simultaneously with the tacking action, the electrostatic field is effective to attract the toner particles comprising the xerographic powder image from the drum surface and cause them to adhere electrostatically to the surface of the support material.

As the paper gripper mechanism continues to move forward in its closed circuit, it will strip the support material from the xerographic drum and carry it to a fixing device,

such as, for example, heat fuser 46, whereat the developed and transferred xerographic powder image on the support material is permanently fixed thereto.

After fusing, the finished copy is preferably discharged from the apparatus at a suitable point for collection externally of the apparatus. To accomplish this there is provided a pair of delivery rolls 47 and 48 by means of which the copy is delivered to a copy holder after it is released by the gripper mechanism. Suitable cam means and 64 are provided at the receiving and delivery stations of the conveyor mechanism to actuate the paper gripper at these stations to receive or discharge a sheet of support material.

The next and final station in the device is a drum cleaning station E whereat any powder remaining on the xerographic drum after the transfer step is removed and whereat the xerographic drum is flooded with light to cause dissipation of any residual electrical charge remaining on the xerographic drum.

To aid in the removal of any residual powder remaining on the xerographic drum there is provided a corona precleaning device 51 that is substantially similar to the corona discharge device that is employed at charging station A. Removal of residual powder from the xerographic drum is effected by means of a web cleaner device 54 adapted to continuously feed a clean fibrous web material into wiping contact with the xerographic drum. As shown, the web material 55 is taken from a supply roll 56' and transported around a cleaning roll 57, preferably made of rubber, around a guide plate 58 to be wound on a take-up or rewind roll 61.

Any residual electrical charge remaining on the xerographic drum is dissipated by light from a fluorescent lamp 62 mounted in a suitable bracket above the xerographic drum, a suitable starter being provided for energizing the fluorescent lamp.

Suitable drive means drive the xerographic drum, the copy drum, and the sheet conveyor mechanism at predetermined speeds relative to each other, and to effect operation of the paper separator roll, and the web cleaner mechanism, the latter being driven at a speed whereby relative movement between the xerographic drum and the web material is effected. Suitable drive means are also provided for eifecting operation of the conveyor mechanism and toner dispenser of the developing apparatus assembly.

It is believed that the foregoing description is suflicient for the purposes of this application to show the general operation of a xerographic reproducing apparatus. For further details concerning the specific construction of the xerographic apparatus shown, reference is made to copending application, Serial No. 77,954, filed'concurrently herewith on December 23, 1960, in the name of Roger H. Eichorn and William G. Lewis.

Fuser The developing material used to form the powder images are specifically chosen to permit them to be fixed to the support material by heat fusing or fixing whereby thepowder particles of the resin or toner used to form the images are softened or tackified when heated so that they readily adhere to the support surface to form a permanent image thereon.

Referring now in particular to FIGS. 2 to 5, inclusive, there is shown a preferred embodiment of a heat fuser constructed in accordance with the invention, which employs two conventional radiant glass panels to supply heat for fixing the powder images onto a support material.

As shown in the drawings the heat fuser 46 includes a radiant bottom panel 501 and a radiant top panel 521 mounted in spaced-apart relation to each other to permit a sheet of support material carrying an unfused powder image thereon to be inserted therebetween for the purpose of heat fixing the powder image on the support material.

The radiant bottom panel 501 includes a base plate 502 having formed integrally therewith side walls 503 and 504 extending along the length of the plate, an end wall 505 extending along part of one end of the plate, depending support members 506 adjacent to the end wall at one end of the base plate and depending support members 507 at the other end of the base plate. A retainer bracket 508 removably secured to the opposite end of the base plate from end wall 505 forms with the base plate an enclosure for a conventional radiant glass panel 510, which is thermally insulated from the enclosure by means of strip insulators 509, such as asbestos, bonded to the walls of the base plate and to the retainer bracket, and by insulator 511, such as a mat of fiber glass, sandwiched between the base plate and the radiant glass panel.

As commonly known, the radiant glass panel 510 has a resistance metallized coating on the back side of the glass which is electrically connected to terminals of a suitable conductor material, not shown, formed at opposite ends on one surface of the panel. Voltage applied to the coating heats the coating and heat is conducted to the glass which then acts as a radiator of heat.

Slotted insulators 512, preferably made of ceramic material or the like, secured to opposite ends of the base plate each movably supports a spring 513 having conductive buttons 514 mounted thereon biased by the spring into electrical contact with the terminals of the radiant glass panel. Each of the springs 513, which serve as a bus bar, is connected by a wire 515 to a suitable source of electrical power.

Paper guides or supports 516, such as wire grids, secured at opposite ends to the side walls through suitable apertures formed therein, extend at an angle to the direction of paper motion across the top of the radiant glass panel to prevent a sheet of support material from coming into direct contact with the radiant glass panel. The guides are preferably placed at an angle to the direction of paper travel so that the paper will only contact the guides momentarily as it travels through the fuser.

The radiant top panel 521, similar in construtcion to the radiant bottom panel, includes a top plate 522 having formed integrally therewith depending side walls 523 and 524 extending along part of one end of the plate, upright support members 526 adjacent to the end wall at one end of the top plate and upright support members 527 at the other end of the top plate. A retainer bracket 508 removably secured to the opposite end of the top plate from end wall 525 forms with the top plate an enclosure for a radiant glass panel 510 thermally insulated from the enclosure by strip insulators 509 bonded to the walls of the top plate and the retainer bracket, and by insulator 529 sandwiched between the top plate and the radiant glass panel.

Slotted insulators 512 secured to opposite ends of the top plate each movably support a spring 513 having conductive buttons 514 mounted thereon biased by the spring into electrical contact with the terminals of the radiant glass panel. Each of the springs is connected by a wire 515 to a suitable source of electrical power.

Power to the fuser is controlled by means of a conventional thermostat 531 inserted through suitable apertures in the top plate 522 and insulator 529 so that it rests in contact with the radiant glass panel of the radiant top panel. The thermostat 531 is held in position by leaf springs 532 secured to the top of the top panel.

The radiant top panel and radiant bottom panel are secured at one end by their support members 526 and 506, respectively, to plate 11 and at their opposite ends by their retainer brackets 508 to plate 10. Plates 10 and 11 may be formed integrally with the main frame of the machine, as shown, or, optionally may be formed as separate plates bolted or otherwise secured to the main frame of the machine.

The top panel is mounted above the bottom panel at a distance therefrom suflicient to permit the paper grippers and the chains or endless belts of the sheet conveyor to travel in the space between the panels whereby a sheet of support material may be transported through the fuser.

To conserve heat within the fuser and to partly prevent heat transfer by convection from the heat fuser to the xerographic drum, the side opening of the fuser nearest to the xerographic drum is provided with a gate 541 to partly close the opening between the radiant top and bottom panels when the fuser is being operated on a standby basis. When the xerographic apparatus is in operation, the gate is opened to permit a sheet of support material to be transported through the fuser by the sheet conveyor mechanism.

The gate 541 secured to a crank rod 542 is pivotally supported by the crank rod in clips 543 secured to the side wall 523 of the top plate nearest the xerographic drum. Stated in other words, the clips are mounted to the front side wall as determined by the direction of travel of the chains of the sheet conveyor. Openings 544 are provided in the gate for the passage of the endless belts or chains of the sheet conveyor.

In normal operation the gate is opened by means of a solenoid SOL-1 mounted on bracket 545 secured on the outboard face of plate 11. A lever 546 guided in the bifurcated turned-up portion of the bracket is connected atone end by pin 547 to the normally extended plunger of the solenoid, and at its other end to the crank rod 542 to actuate the gate. When the solenoid SOL-1 is actuated, its plunger is retracted to actuate the crank rod to rotate the gate inward toward the radiant glass panel of the radiant top panel, this movement being in the direction of travel of the sheet conveyor. A solenoid having a normally extended plunger is used in the preferred embodiment of the apparatus, so that if the solenoid fails to operate, the gate can still be opened by contact of a paper gripper 44 of the sheet conveyor mechanism 43 striking the gate and forcing it open as the sheet gripper travels through the fuser.

In the embodiment shown, plate is provided with an aperture through which the radiant top and bottom panels may be inserted into the machine. To eliminate heat loss from this end of the fuser a closure plate 551 and insulator pad 552 are secured to the radiant top panel and the radiant bottom panel. Although a closure plate is shown only at one end of the fuser it is apparent that the opposite end of the fuser can also be sealed by a second closure plate if so desired.

The radiant glass panels are suitably connected to an electrical circuit, in parallel with each other but in series with the thermostat to permit it to control the amount of electrical energy supplied to both radiant glass panels.

By using two radiant glass panels mounted face to face with each other, uniform heating of both sides of the support material to facilitate fusing of the powder image carried thereby is obtained, and, furthermore, the use of radiant glass panels provides uniform heating of the entire surface area of the support material.

'It is noted that wire grids or supports are not the only way of preventing the entire trailing edge of a sheet from contacting the plate to cause charring. A number of glass surface patterns are commercially available in the form of small prisms on the glass surface which will prevent paper charring by permitting only momentary contact of the paper on the fuser.

It is also apparent that the gate could be loosely hinged,

as shown, whereby the gate would be forced inward by a 2 sheet gripper to permit the passage of a sheet material into the fuser.

While the invention has been described with reference to the structures disclosed herein it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or'thc scope of the following claims.

What is claimed is:

1. Apparatus for fixing a powder image to the surface of a sheet of support material, said apparatus including an endless conveyor mechanism for transporting a sheet in a closed path from a sheet receiving station to a sheet delivery station, a first radiant glass panel and a second radiant glass panel positioned between said sheet receiving station and said delivery station in spaced parallel relation to each other to form a chamber therebetween through which said endless conveyor mechanism can transport a sheet of support material, a gate operatively connected to said first radiant glass panel for movement from a first position in which said gate extends substantially from said first radiant glass panel to said second radiant glass panel to form a movable end wall of said chamber while still permitting said endless conveyor mechanism to operate, to a second position in which s id gate is positioned away from said second radiant glass panel to leave said chamber unobstructed, and means connected to said first radiant glass panel and to said second radiant glass panel for connecting them to a source of electrical power.

2. Apparatus for fixing a powder image to the surfac of a sheet of support material, said apparatus including an endless conveyor mechanism for transportinga sheet in a closed path from a sheet receiving station to a sheet delivery station, a first radiant glass panel and a second radiant glass panel positioned between said sheet receiving station and said sheet delivery station in spaced parallel relation to each other to form a chamber therebetween through which said endless conveyor mechanism can transport a sheet of support material, a gate operatively connected to said first radiant glass panel for movement from a first position in which said gate extends substantially from said first radiant glass panel to said second radiant glass panel to form a movable end wall of said chamber while still permitting said endless conveyor mechanism to operate, to a second position in which said gate is positioned away from said second radiant glass panel to leave said chamber unobstructed, control means operatively connected to said gate for actuating said gate, and means connected to said first radiant glass panel and to said second radiant glass panel for connecting them to a source of electrical power.

3. Apparatus for fixing a powder image to the surface of a sheet of support material, said apparatus including an endless sheet conveyor mechanism including at least one sheet gripper for transporting a sheet in a closed path from a sheet receiving station to a sheet delivery station, a first radiant glass panel and a second radiant glass panel positioned between said sheet receiving station and said sheet delivery station in spaced parallel relation to each other to form a chamber therebetween through which said sheet gripper of said endless sheet conveyor mechanism transports support material, a gate operatively connected to said first radiant glass panel for movement from a first position in which said gate extends substantially from said first radiant glass panel to said second radiant glass panel to form a movable end wall of said chamber while still permitting said endless sheet conveyor mechanism to operate,to a second position in which said gate is positioned away from said second radiant glass panel to leave said chamber unobstructed, the direction of movement of said gate to said second position being in the direction of movement of said sheet gripper through said chamber whereby said sheet gripper can open said gate, control means operatively connected to said gate for normally actuating said gate, and means connected to said first radiant glass panel and to said second radiant glass panel for connecting them to a source of electrical power.

4. Apparatus for fixing a powder image to the surface of a sheet of support material, said apparatus including an endless sheet conveyor mechanism having two endless chains, sprocket wheels for supporting said endless chains to travel in unison in opposed relation in two spaced substantially parallel planes, at least one rear- Wardly facing sheet gripper attached to said endless chains along a line perpendicular to the two planes for transporting a sheet in a closed path from a sheet receiving station to a sheet delivery station, a first radiant glass panel and a second radiant glass panel positioned between said sheet receiving station and said delivery station in spaced parallel relation to each other to form a chamber therebetween through which said sheet gripper of said endless sheet conveyor mechanism transports support material, a gate operatively connected to said first radiant glass panel for movement from a first position in which said gate extends from said first radiant glass panel to substantially said second radiant glass panel to form a movable end wall of said chamber, to a second position in which said gate is positioned away from said second 10 radiant glass panel to leave said chamber unobstructed References Cited in the file of this patent for the passage of a sheet carried by said sheet gripper, UNITED STATES PATENTS said gate having openings therein for the passage of said endless chain whereby said endless sheet conveyor can 1,550,422 Braemer 13, 1923 operate when said gate is in said first position, control 5 2,282,926 Bates May 12, 1942 means operatively connected to said gate for actuating 2,564,706 Mochel 1951 said gate, and means connected to said first radiant glass 2,571,218 Del Butter) 15, 1951 .panel and to said second radiant glass panel for connect- 2,807,704 Allen at P 1957 ing them to a source of electrical power. 2,852,651 Crumrine P 16, 1958 

1. APPARATUS FOR FIXING A POWDER IMAGE TO THE SURFACE OF A SHEET OF SUPPORT MATERIAL, SAID APPARATUS INCLUDING AN ENDLESS CONVEYOR MECHANISM FOR TRANSPORTING A SHEET IN A CLOSED PATH FROM A SHEET RECEIVING STATION TO A SHEET DELIVERY STATION, A FIRST RADIANT GLASS PANEL AND A SECOND RADIANT GLASS PANEL POSITIONED BETWEEN SAID SHEET RECEIVING STATION AND SAID DELIVERY STATION IN SPACED PARALLEL RELATION TO EACH OTHER TO FORM A CHAMBER THEREBETWEEN THROUGH WHICH SAID ENDLESS CONVEYOR MECHANISM CAN TRANSPORT A SHEET OF SUPPORT MATERIAL, A GATE OPERATIVELY CONNECTED TO SAID FIRST RADIANT GLASS PANEL FOR MOVEMENT FROM A FIRST POSITION IN WHICH SAID GATE EXTENDS SUBSTANTIALLY FROM SAID FIRST RADIANT GLASS PANEL TO SAID SECOND RADIANT GLASS PANEL TO FORM A MOVABLE END WALL OF SAID CHAMBER WHILE STILL PERMITTING SAID ENDLESS CONVEYOR MECHANISM TO OPERATE, TO A SECOND POSITION IN WHICH SAID GATE IS POSITIONED AWAY FROM SAID SECOND RADIANT GLASS PANEL TO LEAVE SAID CHAMBER UNOBSTRUCTED, AND MEANS CONNECTED TO SAID FIRST RADIANT GLASS PANEL AND TO SAID SECOND RADIANT GLASS PANEL FOR CONNECTING THEM TO A SOURCE OF ELECTRICAL POWER. 